Processes for separating undesired accompanying substances from technical raw gases by means of physical absorption or gas scrubbing are well known from the prior art. With such processes undesired, acidic constituents from raw synthesis gases generated by gasification or reformation of carbonaceous feedstocks, for example carbon dioxide (CO2) and hydrogen sulfide (H2S), but also further constituents such as carbonyl sulfide (COS) and hydrogen cyanide (HCN), can safely be removed from the desired synthesis gas constituents hydrogen (H2) and carbon monoxide (CO) down to trace amounts. A known and frequently applied process is the Rectisol process, which is described in principle in Ullmann's Encyclopedia of Industrial Chemistry, 6th Edition, Vol. 15, p. 399 ff. In the Rectisol process, the absorption of the above-mentioned undesired disturbing components is effected by cold methanol as absorbent or detergent, i.e. methanol cooled down significantly below ambient temperature, wherein in an absorber column an intensive mass transfer between the raw gas and the detergent is effected. The solubility of the undesired gas constituents drastically increases with decreasing temperature of the methanol and with increasing pressure, while it remains virtually constant for hydrogen and carbon monoxide. Methanol also has the advantage that even at temperatures down to −75° C. it still has a low viscosity and thus good mass and heat transfer properties.
In the Rectisol process, the methanol loaded with the disturbing components, which is used as absorbent, is circulated via regeneration plants. In the regeneration plants, the loaded methanol is liberated from the absorbed gases in a physical way. In a first regeneration step CO2 is removed from the loaded methanol absorbent by depressurization and/or stripping with a gas, for example nitrogen. In a second regeneration step the sulfur-containing gases, COS and H2S, are expelled by heating. It frequently is aimed to produce a COS/H2S gas largely free from CO2, as its economically interesting further processing is impaired by mixing with CO2.
In the Rectisol process, a distinction is made between the standard process and the selective Rectisol process. In the standard Rectisol process, the accompanying gases COS/H2S and the CO2 jointly are separated from the raw synthesis gas in one absorption step. In the so-called selective Rectisol process, the sulfur-containing accompanying gases COS/H2S and the CO2 each are separated from the raw synthesis gas in separate absorption steps proceeding one after the other. This selective absorption becomes possible by suitable adjustment of the process parameters, in particular the quantity ratio of absorbent and gas to be absorbed. The advantage of the selective absorption consists in that the COS/H2S gas and the CO2 gas for the most part already are kept separate during the absorption and only the smaller part must be separated during the regeneration of the methanol.
After passing through mostly several regeneration steps, several partial streams of the methanol liberated from the disturbing components are recirculated to the absorber column. To introduce the required cold into the absorber column, a partly loaded methanol partial stream is guided out of the absorber column, cooled by indirect heat exchange with a coolant in a heat exchanger arranged outside the absorber column, and subsequently recirculated into the absorber column. Cooling of the coolant is effected in a compression refrigeration machine with mostly several compressor stages. Useful coolants include substances such as ammonia or propylene, as it is taught in the German Wikipedia article “Kompressionskältemaschine”, retrieved on 20 Jan. 2015.
When using propylene as coolant under typical operating conditions, temperatures of −44° C. are achieved on the coolant side and −25 to −35° C. on the methanol detergent side.
Since cooling of the detergent represents a significant energy consumer within the Rectisol process, efforts for the energetic optimization of this process also must consider the generation of cold.